Generally, in a commercial electrostatographic marking or reproduction apparatus (such as copier/duplicators, printers, multifunctional systems or the like), a latent image charge pattern is formed on a uniformly charged photoconductive or dielectric member. Pigmented marking particles (toner) are attracted to the latent image charge pattern to develop this image on the dielectric member. A receive member, such as paper, is then brought into contact with the dielectric or photoconductive member and an electric field applied to transfer the marking particle developed image to the receiver member from the dielectric member. After transfer, the receiver member bearing the transferred image is transported away from the dielectric member to a fusion station and the image is fixed or fused to the receiver member by heat and/or pressure to form a permanent reproduction thereon. The receiving member passes between a pressure roll and a heated fuser roll or element.
Sometimes copies made in Xerographic or electrostatic marking systems have defects caused by improper fusing of the marking material or the fuser itself. The incomplete fusing can be the result of many factors such as defects in the toner pressure or fuser rolls. Defects in the fuser rolls can be caused by improper compression set properties resulting from extended use or improper coating of the fuser substrates during manufacture.
A electrographic fuser element includes metallic substrate such as aluminum, an elastomeric cover layer, usually a silicone, and at least one coating over the silicone, generally made of a fluoropolymer such as Teflon (a trademark of DuPont).
This invention and its various embodiments are concerned with the manufacturing process for making these coated fuser elements including fuser rolls and other configurations. While for clarity the term “fuser structure or member” will be used throughout this disclosure and claims, any suitable fusing configurations are intended to be included, such as rolls, belts, and pressure members.
As above noted, fuser rolls used in electrostatographic marking systems generally comprise a metal core cylinder coated with one or more elastomer layers such as silicone. Conventional fuser roll core cylinders are relatively thick walled aluminum alloy cylinders. Such thickness has been desired in order to provide strength and durability as the fuser roll presses against the nip of the adjoining compression or pressure roll. For a 35.00 mm outside diameter fuser roll core, a thickness of 1.5 to 8 mm is fairly standard. Similar dimensions are common in office and production printing systems capable of imaging more than 50 pages per minute. In most embodiments, the fuser roll is provided an outer non-stick surface or covering of polytetrafluoroethylene known as Teflon, a trademark of E.I. DuPont. The Teflon coating can be of any suitable thickness usually on the order of several mils.
The use of a fusing member constructed with a non-stick material as a top layer and a heat resistant base layer has been known in the electrostatographic art. Typical non-stick materials that may be used include polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), perfluoroalkoxy (PFA), polychlorotrifluoroethylene (ECTFE), ethylene-chlorotrifluoroethylene (ECTFE), ethylene-chlorotrifluoroethylene (ECTFE), ethylene-tetrafluoroethylene (ETFE), polyvinylidene fluoride (PVDF) and polyvinyl fluoride (PVF), and blends of these materials.
Fluoropolymer resin by itself, though an excellent non-stick material, is not compliant. Silicone compounds, on the other hand, are compliant. It is known in the art form a fuser member having a material combining the non-stick properties of fluoropolymer resins and the compliant properties of silicone elastomers.
Currently, there are two approaches for making Teflon over silicone fuser members. The first approach is to mold the rubber in between the substrate and the fluoro-plastic sleeve, the other method coats the fluoro-plastic on the silicone and cures it on the silicone. The latter method is preferred because of improved durability and wear. The coating method typically involves the following process: a substrate is primed, silicone rubber is molded, cured and optionally post cured; the required dimension is usually obtained by grinding; the fluoropolymer layers are applied usually by spraying; the whole member is baked above the melting or sintering point (300 C) of the fluoropolymer, then the cured surface is usually polished. The fluoropolymer coating layers typically comprise an adhesive (often silane) and/or polyimide/polyamide coating, an optional fluoropolymer primer layer, an optional mid coat layer and a topcoat fluoropolymer layer that is in contact with the media and toner.
There are several advantages to the latter approach. The fluoropolymer layer can be made thinner than a sleeve, less than the ˜30 μm thick with a tight distribution. Also, there is more choice in the material that can be applied. This configuration has been shown by machine testing to yield a more durable and therefore a longer lived fuser member than the sleeved approach by better resisting damage from paper; stripper fingers or sensors that contact the member.
A problem with this approach is that silicone is degraded at high temperatures, →260° C. while the fluoro-plastic coating develops its best properties when baked in the 370° C. to 400° C. range. There is a delicate balance between under-curing the Teflon which can result in poor wearing components or overheating the silicone and damaging it. The latter condition will cause it to take a set easily when in contact with the pressure roll, stripper fingers or other components causing a PQ defect. In fact, both of these failure modes have been observed indicating there is not sufficient process latitude or process control.
The temperatures required to bake the fluoropolymer layer in a Teflon over silicone system (TOS) damages the properties of the silicone rubber, specifically the compression set properties to the point where set of the member is often seen in the form of flat spots or finger-induced dents. Currently, one may process at a temperature lower than that preferred for Teflon cure to improve set properties at the expense the Teflon properties (primarily wear); the result is degraded life due to occasional poor wear or set. Alternately, the TOS member may be “Molded in Place”, MIP, where silicone rubber is molded in between an extruded Teflon sleeve and substrate. This approach has the disadvantage of a relatively thick fluoropolymer layer with wider dimensional tolerances and less durability.